The fixed type constant velocity universal joint includes a Rzeppa type (BJ) (Patent Literature 1, for example) and an undercut-free type (UJ).
As illustrated in FIG. 13, the fixed type constant velocity universal joint of the Rzeppa type includes: an outer race 3 serving as an outer joint member and having an inner spherical surface 1 in which a plurality of track grooves 2 are equiangularly formed along an axial direction; an inner race 6 serving as an inner joint member and having an outer spherical surface 4 in which a plurality of track grooves 5 are equiangularly formed in pairs with the track grooves 2 of the outer race 3 along the axial direction; a plurality of balls 7 interposed between the track grooves 2 of the outer race 3 and the track grooves 5 of the inner race 6, for transmitting torque; and a cage 8 interposed between the inner spherical surface 1 of the outer race 3 and the outer spherical surface 4 of the inner race 6, for retaining the balls 7. In the cage 8, a plurality of window portions 9 for accommodating the balls 7 are arranged along a circumferential direction.
The cage 8 is held in spherical surface contact with the inner spherical surface of the outer race 3 and the outer spherical surface of the inner race 6. A center of curvature (O2) of a ball center trace line of each of the track grooves 2 of the outer race 3 and a center curvature (O1) of a ball center trace line of each of the track grooves 5 of the inner race 6 are situated symmetrically with respect to a joint center Oj. In other words, the center curvature O1 and the center curvature O2 are offset from the joint center Oj in opposite directions by an equal distance in the axial direction. That is, each of the track grooves 2 of the outer race 3 is offset from the joint center Oj to a joint-opening side by a predetermined distance along a joint center axis X, and each of the track grooves 5 of the inner race 6 is offset from the joint center Oj to a joint-deep-portion side by a predetermined distance along the joint center axis X. Here, the joint center axis X is referred to as a straight line including an axis of the outer race 3 and an axis of the inner race 6 under a state in which an operating angle of the joint is 0°. A joint center plane is referred to as a plane including a center of each of the torque transmission balls 7 and being orthogonal to the joint center axis. The joint center Oj is referred to as an intersection between the joint center plane and the joint center axis.
Therefore, torque-transmission-ball tracks formed by the track grooves 2 of the outer race 3 and the track grooves 5 of the inner race 6 exhibit wedge shapes gradually expanding from one side in the axial direction to the other side. The balls 7 are respectively accommodated in the torque-transmission-ball tracks having the wedge shapes, to thereby transmit torque between the outer race 3 and the inner race 6. In order to retain all of the balls 7 in a joint plane (plane perpendicular to a bisector of an operating angle), the cage 8 is incorporated.
Further, as the fixed type constant velocity universal joint of the Rzeppa type, one having a structure provided with six torque transmission balls has been used as a technical standard for years and supported by many users in aspects of performance, reliability, and the like. However, the applicant of the present invention has developed and already proposed an eight-ball Rzeppa type which is made highly efficient and fundamentally lightweight and compact while ensuring strength, load capacity, and durability equal to or higher than those of a six-ball Rzeppa type as the technical standard (for example, Patent Literature 1 below).
Next, as illustrated in FIG. 14, a fixed type constant velocity universal joint of a UJ type includes: an outer race 13 serving as an outer joint member and having a radially inner surface 11 in which a plurality of track grooves 12 are equiangularly formed along the axial direction; an inner race 16 serving as an inner joint member and having a radially outer surface 14 in which a plurality of track grooves 15 are equiangularly formed in pairs with the track grooves 12 of the outer race 13 along the axial direction; a plurality of balls 17 interposed between the track grooves 12 of the outer race 13 and the track grooves 15 of the inner race 16, for transmitting torque; and a cage 18 interposed between the radially inner surface 11 of the outer race 13 and the radially outer surface 14 of the inner race 16, for retaining the balls 17. In the cage 18, a plurality of window portions 19 for accommodating the balls 17 are arranged along the circumferential direction.
In this case, each of the track grooves 12 of the outer race 13 includes a deep-side track groove 12a in which a ball center trace line in the track groove is drawn as an arc-shaped portion, and an opening-side track groove 12b in which a ball center trace line in the track groove is drawn as a straight portion parallel to an axis of the outer race. A center curvature O2 of the deep-side track groove 12a is offset from the joint center Oj to an opening side of the outer race 13 in the axial direction. Further, each of the track grooves 15 of the inner race 16 includes a deep-side track groove 15a in which a ball center trace line in the track groove is drawn as a straight portion parallel to an axis of the inner race, and an opening-side track groove 15b in which a ball center trace line in the track groove is drawn as an arc-shaped portion. A center curvature O1 of the opening-side track groove 15b is offset from the joint center Oj to an axially opposite side (i.e., deep side) relative to the center curvature O2 of the deep-side track groove 12a of the outer race 13 by an equal distance F.
As described above, in contrast to the Rzeppa type in which an entire track is formed into an arc-shape, a track of the outer race 13 of the UJ type is formed into an undercut-free shape having a straight opening side. Therefore, in comparison with the BJ type, ball positions are situated on a radially outer side at an opening portion, and hence an interference angle between a shaft (shaft to be fitted to the inner race) and the track groove 12 of the outer race 13 is increased. As a result, the UJ type can take a higher operating angle than the BJ type. Further, the track of the outer race 13 of the UJ type is formed into a straight shape on the opening side, and hence a radial movement amount of the balls 17 is increased to the radially outer side. Along with the increase, an outer diameter of the cage 18 is increased for retaining the balls 17. Consequently, a diameter of an inner spherical surface of the outer race 13 is increased.
However, in the UJ type, through increasing the radially inner surface (inner spherical surface) of the outer race 13, the arc-shaped track grooves of the outer race 13 are offset to the opening side, and hence a track depth on the deep side is shallow. Thus, as described above, when the inner spherical surface of the outer race 13 is increased, the track groove depth on the deep side becomes shallower. Here, the track depth is referred to as a distance between a spherical surface and a ball contact point at which a contact ellipse of the ball is closest to the spherical surface when analysis of a joint internal force is performed in a rotating state, the ball moving in the track during one rotation in the axial direction and a contact angle direction.
Further, in view of retaining the balls 7, 17 by the cage 8, 18 and ensuring the track depth, in a case of the same size, the UJ type is larger than the Rzeppa type in ball diameter and pitch circle PCD, and even in outer diameter of the outer race.
The UJ type illustrated in FIG. 14 has a cage offset shape effective in ensuring the track depth on the deep side of the outer race. That is, from the joint center Oj, a center O4 of an outer spherical surface 18a of the cage 18 is offset to an opening side in the axial direction by fc, and a center O3 of an inner spherical surface 18b of the cage 18 is offset to a deep side in the axial direction by fc. Such a cage offset type joint is referred to as a track-direction cage offset type.
In recent years, there has been proposed a joint of an eight-ball UJ type having an outer diameter smaller than that of a six-ball type (Patent Literature 1). The joint of the eight-ball UJ type has a ball diameter smaller than that of the six-ball type. Thus, in order to ensure a radial dimension (thickness) of the cage corresponding to the radial movement amount of the balls determined by PCR and an offset amount irrespective of the size and number of the balls, the offset amount is set to be small, and as illustrated in FIG. 14, cage offset is adopted. Here, PCR is referred to as a length of a line connecting between the center of the ball and an arc center of the track groove of the outer race or an arc center of the track groove of the inner race.
For the eight-ball UJ type described above, an improvement at the time of high angle is important for further improvement of durability.
By the way, conventionally, there has been disclosed a six-ball Rzeppa type, in which a center of the track groove is offset to be positioned away from the joint center axis to a radially opposite side relative to the track groove (Patent Literature 2, Patent Literature 3, and Patent Literature 4).
In Patent Literature 2, the track groove of the outer race includes an opening-side first guide groove having a center corresponding to the joint center, and a deep-side second guide groove having a center offset from the joint center to the radially opposite side. Further, the track groove of the inner race includes a deep-side second track groove having a center offset from the joint center to the deep side along the joint center axis, and an opening-side second guide groove having a center offset from the center of the deep-side second guide groove further to the radially opposite side.
With this configuration, a groove depth of a deep-side first guide groove of the outer race is increased. Further, a thickness of the inner race is increased in the opening-side second guide groove of the inner race. Therefore, when the joint takes a high operating angle, there is eliminated a chip of an edge portion of the groove caused when the ball climbs onto the deep-side first guide groove of the outer race. Further, there is eliminated damage of the inner race caused by the load applied from the ball.
In Patent Literature 3, the center of the track groove of the outer race and the center of the track groove of the inner race are separated from a diameter-direction plane (joint center plane) respectively to both sides in the axial direction by an equal distance, and are offset to be positioned away from the joint center axis to the radially opposite side by a predetermined amount. With this configuration, under a state in which the joint takes the maximum operating angle and the ball is extremely close to an inlet edge portion of the track groove of the outer race, a contact force between the ball and the track groove is reduced. Consequently, damage of the inlet edge portion of the track groove is prevented.
In Patent Literature 4, a center curvature of a groove center line of the track groove of the outer race and a center curvature of a groove center line of the track groove of the inner race are set to be eccentric to both sides of the joint center plane, and to be situated on opposites sides beyond a shaft center on a plane including the groove center line and the shaft center. With this configuration, it is possible to increase the maximum allowable angle for the joint angle, and to ensure strength without increasing the outer diameter of the outer race.
Further, conventionally, there has been disclosed a joint capable of increasing a maximum articulation angle without influencing running characteristics, etc. (Patent Literature 5). That is, in Patent Literature 5, a distance between a base of a running track and an axis of rotation of the joint starts from a point of the maximum value, and an angle of intersection formed between a tangent line of a trace curve and the axis of rotation of the joint increases monotonously.